1. Field of the Invention
The present invention is concerned with a bootstrap circuit used for driving a power MOS transistor in a high side driver configuration.
2. Description of the Prior Art
One configuration for driving a power MOS transistor is the so-called "high side driver", in which the load is driven from the high side. Driving circuits can be implemented using discrete devices as well as integrated circuits. Operation of such driving circuits at high frequencies is limited by the threshold voltage of the power MOS transistor. As is known in the art, this threshold voltage is the minimum voltage which must be applied between the gate and the source of the power MOS transistor itself so that current can begin flowing between the drain and the source.
A widely used technology, for example, provides power MOS devices having a threshold voltage of about 3.5 volts. In order to obtain reliable operation of such a device as a switch, it is necessary to apply at least twice the threshold voltage between the gate and source. In the case of a device having threshold voltage of 3.5 volts, 7 volts is thus required for reliable operation.
Bootstrap circuits realized using conventional techniques allow a voltage equal to the supply voltage, less the losses introduced by the circuit itself, to be applied between the gate and the source of the power MOS transistor. It follows that, in order to achieve reliable operation of the power MOS transistor, the minimum value of the supply voltage to the bootstrap circuit must necessarily be higher than 7 volts.
Two classic circuit configurations are known in the art for driving a power MOS transistor: the bootstrap, which operates at a frequency equal to the switching frequency of the power MOS transistor; and the charge-pumping circuit, which operates at a frequency much higher than that of the power MOS transistor. Examples of these circuits are described in the article, "Drive Techniques for High Side N-Channel MOSFET's" Warren Schultz, Motorola Inc. SPS, PCIM, June 1987.